Low tension spark plugs



1, 1956 G. D. SUTER 2,760,099

LOW TENSION SPARK PLUGS Filed May 4, 1955 m J r 25 VENTOR.

l2 GEORGE D. wra 34 A 9 4m 40. W

A-r-roauav Low TENSION SPARK PLUGS.

George D. Suter, Wauwatosa, Wis., assignorto Globe- Union Inc.,Milwaukee wis a corporation of Delaware Application May 4, 1953, SerialNo. 352,767

1 Claim. or. 313-131 This invention relates to improvements in lowtension spark plugs.

Such plugs operate at relatively low voltages, in the neighborhood of1000, require small gaps, say from approximately .0O2" to .015", anddischarge a relatively large amount of current, say about 400 amperes,inthe short time of about A. to 1 /2 microseconds. Under such conditionsthe sparking faces of the electrodes rapidly nited States Patent erodewhich either changes the spacing of the spark gap or causes the spark torecede and thus quickly cause the plug to lose its specified operatingcharacteristics.

It is an object of this invention, therefore, to provide a spark plug ofthis type which will retain substantially the same spark gap andmaintain the spark at its operating level even though erosion takesplace during use.

Another object of this invention is to provide a spark plug having theforegoing characteristics which is relatively inexpensive to manufactureand may be made within close tolerances and standards of performance.

These objects are obtained by providing electrodes with sparking faceabridged by the spark guiding face of a gap forming insulator andarranging all such faces so that as erosion takes place on one sparkingface a new sparking surface will be exposed in the plane of the facethereof and the other sparking face, together with the spark guidingface, will recede along a line parallel with the plane of such firstface. The electrode providing the receding face has a thickness within arange which will permit the entire edge which constitutes the sparkingface to recede and leave no overhanging lip behind which the spark mayoccur without fuel ignition. The sparking face along which erosion takesplace at newly exposed places in the plane of the face is used todetermine the optimum level of the spark with physical relation to thecombustion chamber. This level and the spark gap will not be changedduring use by the action of erosion.

The novel features, which are considered characteristic of theinvention, are set forth with particularity in the appended claim. Theinvention itself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description of a specificembodiment when read in connection with the accompanying drawing, inwhich:

Fig. 1 is a view partly in side elevation and partly in section of a lowtension spark plug embodying the present invention;

Fig. 2 is a bottom plan view of the plug of Fig. 1 with parts brokenaway and shown in section;

Figs. 3 and 4 are fragmentary sectional views of the lower end of theplug illustrating diagrammatically the manner in which the sparkingsurfaces erode; and

Figs. 5 and 6 are views similar to the Figs. 3 and 4 illustrating bycontrast the effects of erosion if the center electrode is thicker thannecessary.

Referring to the drawings, the spark plug exemplifying this inventionhas a metal shell 10 of conventional design functioning as the outerelectrode. The bottom of the shell constitutes a sparking face 12 onsuch outer electrode. Such'face lying in a plane normal to the axis ofthe plug determines the level at which the arc will take place withrespect to the plug and any combustion chamber in which the plug isseated in normal fashion. A ceramic core 14, also of conventional designwith an.in-. ternal bore, is held within the shell 10 against a. gasket16 by sealing material 18 and has its bottomsubstantially flush with thesparking face 12. i I 1 A center electrode consists of a thin disc-likehead or member 20 which has a stem 22 extending upwardly into the boreof the core 14 and threadedly connected toa stud 24 extending in suchbore from thetop of the core 14. A gasket 26 at the lower end of stud 24provides a seal for the stem 22. The upperside of the member 20 isspaced from the sparking face 12 by a layer of insulating material 28which is coextensive with such upper side. This layer thus determinesthe amount of the spark gap. The edge of the member20 extending betweenthe sides of such member constitutes a sparking face 30 on the centerelectrode. The sparkor arc jumps from the sparking face 30 to thesparking face 12, being guided by the edge of the insulating material28.

Contrary to some theories now being advanced concerning the necessityand advantage of building up carbon deposits on the spark guiding faceof the insulating material, it has been discovered that for bestoperation of a low tension spark plug, the gap should be as small as iscommercially feasible so that the required spark over voltage will beless than the applied low tension charge, and the spark guiding surfaceof the insulating member should be as free as possible of carbonparticles, combustion residues, and other conductive materials.Therefore, in the plug herein described the thickness of the insulatingmaterial is not greater than .010 inch.

Various types of insulating material may be used. One type consists of avery thin layer of glass (can be under .010 inch thick). Such layer maybe fused to one or both of the electrode 20 and shell 12. It is notaffected by the subsequent sealing of such electrode. The spark betweenthe sparking faces 12 and 30 creates enough heat to melt and erode theedge of the glass layer as fast as the sparking face 30 erodes towardthe center of the plug. Another type of insulating material consists ofthin disc-like sheets of mica placed over the stem 22 and insertedbetween the head 20 and the bottom of the shell 10 and core 14. Caremust be taken to get mica which is low in impurities to resist interiorbreakthrough and form conductive paths inwardly of the spark guidingedge of the mica.

The thickness of the electrode 20 will vary depending upon themetallurgical composition thereof. The essential characteristic is tomake it thin enough so that substantially the whole surface of its edgewill erode inwardly evenly. In Figs. 3 and 4 the electrode 30 is thinenough so that it eroded evenly. A new receded sparking surface formedby such erosion is indicated at 32. With conventional nickel alloy thethickness should be approximately .015 inch. If the thickness of theelectrode 20 is too great as diagrammatically shown in Figs. 5 and 6,the erosion will not extend over the area of the edge and thus will tendto leave an overlying lip as indicated at 34. This blankets the sparkwhich recedes between the.

sparking faces and destroys its efiiciency.

Hence as erosion takes place the spark or are will move inwardly alongthe sparking face 12. As pits or valleys form in the surface of the face12 by erosion the Patented Aug. 21, 19 56 3 always determines'thedevelofi the arc with respect to the plug (and consequently. the combustionchamber-) andhence such level does not vary during the useful life ofthe plug. This, together with the fact that the gap (and consequentlythe spark) remains substantially con staut,-makesftl1e=plug of constanteflicien'c'y throughoutitsispan ofausefulnesst The use: of a fiatbackingrsurface (-upper sid'e: of electrode 20): for the insulatingmaterial permits such layer: to: be extremely" thin and: still retainsufi'icient mechanical strengthv and dielectric properties; In 1somerinstances gaps: as small as'1002 inch may there fore: be: obtained;

Although only one embodiment of the invention: is shown and described:herein,- itv will: be understood that thisiapplicationiis intended t'ocover suchchanges or 'Iil'ddl ficati'ons' as come withim the: spirit ofthe: invention or scope :05 the following cla'un r A low tension: sparkplug lli'aving a: metal shell e'le'cingz.facerdetermining thelevel ofthe are;- an internally bored insulating; core: having: a lower end'substantially evemwithsaid sparking f surfaew a second ele'c trod c'onirisingwt thiniflat sided member lia'ving it's sides arallel to andin=overlappingirelationship with said fl'rst 'spa'rking surface and anedge constituting a second sparking face, thethicknessof-saidsecondelectrode beingmsrnall enough so that erosion onthe sparking face edge thereof recedes the edge over its full distancebetween the sides to prevent occurrence of overhanging lips which lessenthe effectiveness of the arc, a layer of insulating material not greaterthan .010 inch-thick-between the upper side of said second electrode andsaid first sparking surface and said lower end; said layer? determiningthe spark gap, said insulating material being of a character which WillBurn away at a" rate c'tiiia'l t'o therate of erosion of said'seconds'parkihg'fa'c'e; tlie edge of said layer Being parallel to and forminga straight extension of saidfirst mentioned edge and: constituting asparkguiding smooth bridge between said sparking. faces, and a studsecured to said second electi'odearfd'extending through and sealed inthe bore of said core.

References-Gited-imthefile of this-patent= UNITED STATES PATENTS

